Fiber-optic sensors based on Sagnac interferometric circuitry described herein refer mainly to two kinds of sensors, e.g., gyroscopic rotation-rate sensor, or gyro for short, working on the Sagnac effect, and electric current or magnetic field sensor working on the Faraday effect. The essential likeness of these two effects is that both are nonreciprocal, thus leading to the circumstance that the two relevant fields, gyroscope and current sensor, have been developed parallelly on the common basis of Sagnac inteferometric fiber-optic circuitry. As a matter of fact, the R&D and T&D (T: Technology) of current sensor is largely benefited by borrowing abundance of useful technologies from its forerunner, the gyroscope.
The past decades have witnessed admirable successes in practical applications of gyroscopic sensors employing active bias, e.g., piezoelectric phase modulator (PZT) or integrated optics chip (Y-waveguide junction). Following this pace, actively biased current sensors have also been explored for a fairly long time, but the advance of this art is less successful. One major reason for the lagging behind in the R&D of current sensor is attributive to the fact that, while an all-fiber gyro architecture can be constructed in its entirety by employing the well-developed linear PM (polarization-maintaining) fiber, or so-called hi-bi fiber, a phase-detection-based current sensor of high accuracy and long-term stability needs, at least for the part of its sensing loop, the use of circular PM fiber that is by far less developed. To emphasize the specialty of circular polarization-maintaining in contrast to linear polarization-maintaining, italic style of the word “circular” is employed here and afterwards whenever it appears in connection to such special kind of PM fiber.
Despite the said successes of actively biased fiber-optic Sagnac interferometric gyros, efforts have never been discontinued in attempting to passively bias an interferometric circuitry adaptable to either gyro or current sensor applications. Attractive aspects of passive biasing include simplicity and robustness in construction, lower noise, lower cost, easier adjustment (oftentimes no need of adjustment, with the desired system behaviors accomplished all automatically), higher accuracy and stability in long-term operation, etc.
Early attempts to realize passive biasing of fiber-optic sensors employed a 3×3 coupler made of three conventional single-mode fibers serving as a beam splitter and meanwhile a passive bias in a Sagnac interferometric fiber circuit. A major deficiency of such fiber-optic interferometric architecture is that such coupler made of single-mode fiber is not capable of maintaining the SOP (State of Polarization) of light, thus incurring instability of lightwave transmission in the circuitry comprising the coupler. A 3×3 linear hi-bi fiber coupler is capable of sensing, as well as maintaining, a gyroscopic signal. Yet, up to the time being, fabrication of 3×3 linear hi-bi fiber coupler poses a serious difficulty in the process of paralleling three sets of mirror-symmetric stress-elements in constituent linear hi-bi fibers of the coupler. A search of worldwide market information on optical fiber products shows that only a very few companies provide commercial products of 3×3 linear hi-bi fiber coupler. Laboratory use of such products from one company gave extinction ratios substantially lower than 20 dB for linear light in the coupler arms, and thus could not be acceptable for use in practical sensor applications.
Another way to passively bias a fiber-optic Sagnac interferometric sensor circuitry is to employ a nonreciprocal Faraday rotator. Complexity now arises regarding how such Faraday rotator is included in the fiber-optic sensor circuitry. Presently available fiber-optic circuitry is essentially based on linear hi-bi fiber, yet a Faraday rotator works on circular light in order to acquire a phase change for accurate phase-detection. Existing method in actual sensor circuitry is either to use a bulk-optic quarter wave plate, or a short linear hi-bi fiber section with a 45° excitation, in order to provide the required linear  circular SOP transformation. However, the former method is unfavorable for the need of greatly increased structural complexity, while the latter is imperfect for the inherent length-sensitive behavior of the said linear hi-be fiber section.
It is on the said technical background that the present invention comes into being with a view to overcome the existing difficulties and complexity in present fiber-optic sensor technology using one or the other kind of the above-described passive bias. Technical particulars of the invention is given below in Summary and Detailed Description of the Invention in connection with the accompanying figure drawings.